The invention relates to a method and a device for assessing the damage to rolling bearings of electric machines, in particular of inverter-fed electric machines.
Parasitic effects, which include a flow of current via the bearings of the respective motor, arise in electric machines, in particular in motors with inverter feeding.
With larger electric motors which are connected directly to a sine-wave electrical network, bearing currents occur in particular which are caused by asymmetries of the electrical circuit, manufacturing tolerances and material anisotropies. The asymmetrical distribution of the magnetic flux in the motor induces a voltage in the shaft of the electrical machine which leads to a low-frequency current flow through the bearings. These currents circulate in a closed circuit of shaft—bearing—bearing plate—housing.
Interruption of this current flow is achieved by isolating the bearing.
In electrical machines with electrical feed by an inverter, especially an intermediate circuit voltage converter, the output voltage is generated by regulated switching of the direct current intermediate circuit, which is then present at the output of the inverter. A switch from positive and negative potential in a rapid sequence leads in a two-point inverter to a voltage curve of which the sum of the three-phase current is not equal to zero and is referred to as common-mode voltage.
Each of these steep current switching actions causes high-frequency excitations which lead to high-frequency harmonic waves with the currents resulting therefrom which flow back via parasitic paths to the source, i.e. to the inverter voltage link.
These currents can cause changes to the movement path in the bearings, especially in the rolling bearings. With major changes to the movement path the rolling bearings initially cause noise when rolling over the surface. The bearing then fails later with fatigue damage, which can lead to the failure of the electrical machine or to it sustaining damage.
The bearings involved thus have to be changed before they reach the desired bearing lifetime, which results in unplanned costs.
For motors embedded into systems an electrical bearing load is deduced from external measurements. In such procedures an attempt is made, by measuring the ground currents, shaft currents and shaft voltages or by making noise measurements, to produce a state description of the bearing. This state description is extremely imprecise however. Therefore, for the sake of safety, the bearings have to be changed long before they actually fail.
On this basis, the underlying object of the invention is to create a more exact method compared to the previous noise and comparison measurements to enable the remaining lifetime of a bearing to be estimated more accurately.